Ethylene (co)polymers, which contain groups derived from vinylsilanes, are used commercially for many purposes, for example for electrical wire coating (jacketing and/or insulation layer(s)) of various types, pipes, adhesives, gasketing, and crosslinked foams. In many of these applications the silicon atom has attached to it one or more groups which can hydrolyze and then react to form a crosslinked polymer or a covalent bond to a surface.
Generally speaking the vinylsilane groups are “attached” to the ethylene polymers by one of two methods. In the first method ethylene and the vinylsilane are copolymerized at high temperature and pressures, often in the presence of a free radical generator to form a copolymer. See for instance U.S. Pat. No. 3,225,018, which is incorporated by reference herein for all purposes as if fully set forth. Some of the repeat units derived from the vinylsilane in such a copolymer are
wherein Si≡ is a silyl group, and most of the rest are present as ends of chains or there are typically 2 or 4 or more methylene groups between the silicon atom and the methine carbon atom. Since this polymerization process is very similar to the so-called high pressure homopolymerization of ethylene, these copolymers have structures similar to low density polyethylene (LDPE). For an analysis of branching in LDPE, see Usami, et al., Macromolecules, vol. 17, p. 1756 et seq., (1984).
The other method of making ethylene polymers is to graft an allyl- or vinylsilane onto a preexisting polyethylene (PE). See for instance U.S. Pat. No. 3,646,155, which is also incorporated by reference herein for all purposes as if fully set forth. Grafting has the advantage that any type of PE, such as LDPE or high density polyethylene (HDPE) may be grafted. The disadvantage is that the grafting step requires the use of free radical catalysts, and is an extra step in the manufacturing process of the vinylsilane containing PE. Too little free radical catalysis and the vinylsilane is not grafted or only partly grafted, while too much free radical catalysis can result in crosslinking of the PE by conventional free radical methods. The grafted vinylsilane is usually considered to be present as a side chain on the PE having the formula —CH2CH2Si≡ (XIV), where the free bond to the carbon atom is the attachment point to a carbon atom of the PE chain. Similarly, one would expect that grafted allylsilanes would be present as a side chain of the formula —CH2CH2CH2Si≡ (XIX).
PEs containing attached vinylsilanes are particularly useful because they are thermoplastics which may be formed by normal thermoplastic processing techniques until they are exposed to water, and usually also one or both of heat and catalysts for the crosslinking reactions (see for instance previously incorporated U.S. Pat. No. 3,646,155 and U.S. Pat. No. 3,225,018). Once they are crosslinked they typically behave as thermoset materials. More importantly certain properties, such as resistance to treeing (failure brought on by exposure to high voltage electrical fields), and/or stress crack resistance, and/or creep resistance, are usually improved. Thus new methods of preparing allyl- or vinylsilane containing polyolefins, especially polyethylenes, and new vinylsilane containing polyolefin compositions, are constantly being sought.